The present disclosure relates to parallel resonant circuits utilized, for example, for tuner systems.
Tuner systems receive transmission signals of a plurality of channels, select desired channels, and demodulate the signals of the desired channels. Such tuner systems need to have low distortion characteristics. For example, Japan's Integrated Services Digital Broadcasting for Terrestrial Television Broadcasting (ISDB-T) has in total 40 channels, from Channel 13 (473.143 MHz) to Channel 52 (707.143 MHz). Each channel has a signal bandwidth of 6 MHz. Tuner systems also need to have disturbance wave characteristics of 50 dBc or more in accordance with an input level of an interfered channel at each received channel.
In order to achieve such reception characteristics, most tuner systems remove interfering waves using a parallel resonant circuit having variable center frequencies according to received channels at a low noise amplifier at a first stage. On the other hand, low power consumption is also important for tuner systems for mobile terminals.
A typical amplifier circuit includes, as a load of a transconductance amplifier, a parallel resonant circuit formed by connecting an inductor, a variable capacitor, and a variable resistor in parallel. The variable resistor is formed by connecting a plurality of branches in parallel. Each branch includes a series circuit of a resistor and a MOS switch. The variable capacitor is also formed by connecting a plurality of branches in parallel. Each branch includes a series circuit of a capacitor and a MOS switch. See, for example, Y. Kanazawa et al., A 130M to 1 GHz Digitally Tunable RF LC-Tracking Filter for CMOS RF Receivers, IEEE Asian Solid-State Circuits Conference, Nov. 2008, pp.469-472.